You could run the 2.5" converters, but everything in front of and behind the converter (until the pipes join) is 2.25". It would be easy enough to change everything behind the converter to 2.5", but I'm not sure that it would be worth it unless you increased the pipe diameter (headers?) in front of the converters. I think that M Wong (E38 RF) went with the 2.5" converters, but had trouble down the line. See if he has anything else to provide.

Have you considered measuring backpressure while under load to see where you are currently at?